The Welsh language uses a form of the Latin alphabet, just like English does. When the English invaded Wales during the Middle Ages, they brought their alphabet with them. Previous to the invasion Welsh had no written language. However, Welsh itself is older than the English language.
The Welsh language does not use certain letters found in English, and has some traits of their own. The letters k, v, x, and z are not used in Welsh. Very occasionally they will show up in loan words like “kilogram,” but are more often replaced with the Welsh sound-a-like letter. For example, “kilogram” is written “cilogram”.
Just because the letters themselves may be like English’s doesn’t mean that Welsh pronunciation is just like English’s. A good example is the letter j. A j in Welsh makes a sound like the letter t in water or the letter g in garage.